The present invention finds application in the field of paging and, in particular, the establishment of synchronisation between two transceivers. The invention relates to a method of paging, a transceiver for paging and a transceiver for being paged.
In a radio communication network a master transceiver may communicate to one or more slave transceivers. To enable communication links the master should be able to individually address each slave transceiver and to synchronise with the slave transceivers. It is important if frequency division multiplexing is used that a communication link is in a frequency channel shared between master and slave (frequency synchronisation). It is important that if time division multiplexing is used that the communication link uses a timing shared between the master and slave (timing synchronisation).
A paging routine is typically used to establish a communication link between a slave transceiver and the master transceiver. A paging routine typically involves: the slave transceiver being controlled to receive radio signals at a first receiving frequency; the master transceiver transmitting a paging message addressed to the slave transceiver at a first transmission frequency equal to the first receiving frequency; and the slave transceiver replying with a message addressed to the master transceiver at a second transmission frequency. A problem arises when the slave transceiver changes its first receiving frequency. In this case, the master transceiver may not be able to match the first transmission frequency with the first receiving frequency. It is therefore necessary for the master transceiver to transmit with different first transmission frequencies.
One proposal suggests that the master transceiver should transmit a paging message in a transmission time slot and then wait for a reply in an immediately following receiving time slot, for each of a sequence of different transmission frequencies. The frequency at which a reply is transmitted by a slave is determined by the frequency at which the preceding message received by the slave, was transmitted by the master. An advantage of the proposal is that the rigid time slot structure provides for simple timing synchronisation. The reception of a single message by a slave allows for timing and frequency synchronisation with the master. One problem with the proposal is that it may take an unnecessary amount of time for the master transceiver to transmit at the correct transmission frequency, that is, the one at which the addressed slave transceiver is receiving. The master transceiver has as much transmit activity as receiving activity thus half the air time is reserved for reception although only a single slave is paged.
It would be desirable to improve how synchronisation is established during paging.
According to one aspect of the present invention there is provided a transceiver for paging a device by periodically transmitting a series of messages, comprising: transmitter means arranged, when enabled, to transmit a series of messages comprising a message at each frequency in a predetermined series of frequencies; receiver means arranged, when enabled, to receive at, at least a predetermined frequency; and control means arranged to enable said transmitter means and then said receiver means in each of a plurality of successive periods and to provide each message in said series of messages.
The predetermined series of frequencies may be a series ordered in dependence upon the frequencies in the series. Each frequency in the series may be associated with an index and the ordering may take place on the basis of the index.
A first type of message in the series of messages may define when the receiver is enabled in that period. A second type of message if present in the series of messages may define the predetermined frequency for that period. In the preferred embodiment, in one period, there is a multiplicity of messages of the first type transmitted. One, none or more than one message of the second type may be transmitted in a period.
A message in the series may be of the first type, the second type or be both the first and second type. Messages of the first type and the second type may be differentiated using different preambles for transmitted radio packets addressed to the device.
According to another aspect of the present invention there is provided a transceiver, for being paged by a paging transceiver which periodically transmits a series of messages comprising a message at each frequency in an ordered sequence of frequencies and then receives at, at least a response frequency, comprising:
receiver means arranged to receive a message transmitted at a receiving frequency;
transmitter means for transmitting a message at a transmission time with a transmission frequency; control means arranged to control said transmitter means to establish timing and frequency synchronisation between said transmitter means and the paging transceiver, said control means being responsive to the receipt of a message in said ordered sequence to determine at least one of said transmission time and said transmission frequency.
According to a preferred embodiment of this paged transceiver, the control means is responsive to the receipt of a message of a first type, and in particular the receiving frequency on receipt of the message, to determine the transmission time and responsive to the receipt of a message of a second type, and in particular the receiving frequency on receipt of the message, to determine the next transmission frequency.
According to a further aspect of the present invention there is provided a method of paging a device using a paging transceiver, comprising: the steps of periodically: transmitting a series of messages comprising a message at each frequency in a predetermined sequence of frequencies, said series of messages being ordered in dependence upon the frequencies at which they are transmitted; then receiving at, at least a predetermined frequency; and then changing said predetermined frequency until a message is received.
According to a preferred embodiment of this method the series of messages comprises first messages and, if the predetermined frequency is a part of the series of frequencies, comprises a second message transmitted at the predetermined frequency. The predetermined sequence of frequencies may be determined by:
estimating the frequency at which the device will be receiving; calculating the next most likely frequencies at which the device will be receiving; and creating a sequence from said estimated and calculated frequencies.